Pole piece centering arrangement for permanent magnet electrodynamic loudspeakers



Oct. l0, 1950 a. A. ENGHOLM 2,524,780

POLE vPIECE CENTERING ARRANGEMENT EoR PERMANENT MAGNET ELECTRODYNAMICLouDsPEAm-:Rs

Filed July 7', 1945 l ii APatented Oct. 10, 1 950 UNITED STATES PAT srcme-E Y POLE PIECE CENTERING ARRANGEMENT FOR `PERMANENT MAGNET:ELECTRODY- VNAMIC LOUDSBEAKERS Bernard A. Engholm, Cleveland, Ohio,vassignor, by mesne assignments, Lto The 'Rola Company, Inc., .Cleveland,Ohio, a corporation of Ohio Application J lily v7,1945, SjerialNo.v603,605

i3 Claims;

w `hin which theaforesaid coil is supported for operation. f Y

In prior magnetic assemblies lthe :pole tipfhas been held onto the backframe Vby a screw that would extend through the back of the frame andinto the pole-tip, or the tip wouldbe soldered lto the frame, vbuteither operationl is diiiicult to attain hand at vthe same time jte.maintain the desireddegree of eiiic'iency of operation.

One of the objects of 'the present invention is to make an assemblywhich includesl the magnetic' elements as .aforesaid Without the...necessity `for using a-screw or solder 'for holding themagnet andpoletip'inplace. The invention contemplates resilient means for centeringthe magnet and pole tip in proper position and for exerting springtension thereon to maintain Ithe desired relationship between therespective parts of` the assembly. Y An additional object of the presentinvention is to protect the air gap against the entry of foreignparticles which would interfere with the accuracy of voice reproduction.Such additional object contemplates the construction of a magneticassembly as aforesaid, and the prevention of access of foreign particlesto the air gap from inside the magnet structure. The invention is thuswell adapted for installations employing a permanent magnet.

My invention as hereinafter described provides at once the means forpreventing access of particles to the air gap from the region within themagnetic frame, and the means for holding tl-ie`separate magnetic polepiece in place. The invention may be summarized as comprising the meansby which I accomplish these objects, or either of them. A preferred formof such means is illustrated in the drawings hereof and hereinafter morefully explained.

In the drawings, Fig. 1 is a diametric section partly in elevationthrough a loud speaker embodying this invention; Figs. 2. 3 and 4 aresections taken on planes indicated by the correspondingly numbered linesin Fig. 1.

In each of the views, I have shown an open rectangular magnetic framecomposed of two There is an channel-shaped members II and I2 with theiredge flanges secured together as, for instance, by straps I3 spot-weldedto the anges. The web 9-of the member l2- commonly known as a magnetframe Yhas a central opening Iliand into this extends a pole piece l5abutting a permanent magnet I 6 which is suitably secured, as by cementIl, to the web of the member ll. There is thus provided a permanentmagnet within an open frameand having a central pole piece extendinginto an opening in the frame with an annular air gap between them.

' In Fig. 1, 2U indicates the housing or frame of the cone 2l which isof suitable vibratory material to hdeliver sound. The housing 20 has acentralportion 22 which is shown as riveted to theweb of the channellmember I2. The outer edge Vof the cone is' secured to thehousing 20 inthe'usual manner. The cone 2l carries at its apex the voice coil form 25Vabout which is placed a suitable coil 26 which operates within the airgap I4.

Within the frame of the magnet and surrounding the! air gap at somedistance from it, I provide a hollow cylindrical member or thimble 3S ofnon-magnetic material which is suitably secured 'to the web l2 of themagnet frame. Bearing against the outer peripheral edge of this drum isan annular disc 35 having spring-like characteristics, the innerperiphery of which bears against the pole piece I5 of the magnet. Thedisc 35 is flanged at its outer edge toward the adjacent wall of theframe so that it is retained in place by overhanging the thimble 3U. Thepole piece I 5 is preferably made conical, as shown at I9 and the innerperiphery of the d isc 35 bears against such conical surface, this discbeing slightly flexed inwardly when the parts are assembled. Thus thedisc exerts a constant pressure against the pole tip I5 to maintain itin central position with respect to the opening I4.

It will be seen that the simple arrangement of the drum and discdescribed provides at once the means for positioning the separate polepiece and the vmeans for preventing access from the region about it tothe air gap.r

I have found it very satisfactory to make the non-magnetic thimble 30 offormed zinc and the resilient disc of Phosphor bronze and to secure thedrum in place by cement, shown at 3|, but the invention is not limitedto those particular materials. it may also be cemented to the part I6.

In Figs. 2 and 3, I have shown the voice coil spider as surrounded by adust protector. Briefly,

After the pole tip has been centered,...`

such construction comprises an annular cup-like member 40 secured to themagnet frame 9 and having radially extending ange 4| to which the outerportion of an imperforate spider 42 is attached. The inner end of thespider is shown as having a rcylindrical flange l43 which is rigidlyattached as by cement to the neck of the cone 2 I. A closure disc 45 isshown as being attached to the inner face of the cone for sealing theouter end of the voice coil form.

An advantage of the present invention is the fact that constant pressureis exerted against the pole tip to maintain it in centralized positionwith reference to the aperture in the front plate of the magneticstructure. A further advantage is the fact that the aforesaid means alsooperates to protect the air gap from the entrance of foreign particles.

I claim:

1. A permanent magnet loud speaker comprising an open frame havingspaced walls, a central magnetic member secured at one end to the Wallof the frame, a separate pole piece laterally shiftable on the other endof the magnetic member and having a conical head, the larger end ofwhich engages the magnetic member and the smaller end of which joins acylindrical portion which extends into an opening in the opposite wallof the frame leaving a surrounding air gap, a protecting devicesurrounding the separate pole piece and secured at one edge to the Wallof thev frame having the opening and having a portion under constraintconstantly pressing axially on the conical portion of the separate polepiece to maintain the same centered in the air gap.

2. A permanent magnetic loud speaker comprising an open frame havingspaced walls, a central magnetic member secured at one end to the wallof the frame, a separate pole piece laterally shiftable on the other endof the magnetic member and having a conical head, the larger end ofwhich engages the magnetic member and the smaller end of which joins acylindrical portion which extends into an opening in the opposite wallof the frame leaving a surrounding air gap, a thimble surrounding thepole piece and secured at its outer edge to the wall of the frame having4 the opening, and a flexible disc having a central opening surroundingthe separate pole piece and bearing at its inner edge against theconical portion thereof, said disc bearing against the thimble andconstrained thereby to press against the conical surface of the separatepole piece.

3. A permanent magnetic loud speaker comprising an open frame havingspaced walls, a central magnetic member secured at one end to the wallof the frame, a pole piece on the other end of the magnetic member andhaving a conical head, thelarger end of which engages the magneticmember and the smaller end of which joins a cylindrical portion whichextends into an opening in the opposite Wall of the frame leaving asurrounding air gap, a thimble surrounding the pole piece and secured atits outer edge to the Lwall of the frame having the opening, andaflexible disc having a central opening surrounding the pole piece andbearing at its inner edge against the conical portion thereof, said discoverlapping the inner end of the thimble and constrained thereby topress against the conical surfaceof the separate pole piece, the discbeing flanged at its vouter edge in the direction toward the frameportion having the opening to prevent the shifting of the disc upon thethimble.

. BERNARD A. ENGHOLM.

' REFERENCES CITED The following references are of record in the leof'this patent:

UNITED STATES PATENTS Number .Y

